Fix 10.9 Ruby.framework detection/compilation

Mac OS X Mavericks ships with Ruby.framework 2.0 which requires some fixes to enable proper header inclusion and version defines.

This is a stripped down version of the homebrew formula patches at felixbuenemann/homebrew@81824e31564538e9df4d0552cf5063691f17c688.
In comparison to the homebrew patches it only allows compilation against the current Ruby.framework (1.8 on pre-10.9, 2.0 on 10.9).

While the more extended patch for homebrew allows to compile against 1.8 framework on Mavericks, I chose to go with this simpler version because I don't think it makes much sense to link against an ancient unsupported ruby version.